A common known procedure for determining the amount of an antigen in a sample is to allow the antigen in the sample to compete with a distinguishable analog of the antigen for sites on an antibody which is highly specific for the antigen. This general method of determining the amount of antigen present is known as a competitive system. For example, if the amount of digoxin present in the serum is to be determined, an aliquot of the serum and an aliquot of radioactively labeled digoxin is allowed to react with an antibody specific for digoxin. The labeled and unlabeled digoxin compete for sites on the antibody. Thus, the greater the amount of digoxin in the sample, the lower will be the radioactive reading on the antibody.
Two common methods of performing such competitive determinations are known as "liquid phase radioimmunoassay" and "solid phase radioimmunoassay". In each system the immunogenic substance which is bound to an antibody must be removed from unbound immunogenic substance in order to make a measurement of the amount of labeled analog on the antibody. From this measurement, the amount of immunogenic substance in the sample is determined.
In "liquid" phase radioimmunoassay systems, the antibody, labeled analog and immunogenic substance to be determined are incubated in solution. A number of antibody binding sites are available and reaction time is rapid. To separate the antibody complexed immunogenic substance from the uncomplexed immunogenic substance, the antibody immunogenic complex is precipitated, centrifuged, and the supernatant is decanted.
"Solid phase" radioimmunoassay systems avoid the precipitation step. The antibody is bound to a glass chip or inside surface of a tube. Centrifugation of the glass chips or decantation of the coated tubes will separate the antibody complexed immunogenic substance from the uncomplexed immunogenic substance. However, the reaction time is relatively slow because the number of available active sites on the antibody is blocked by the chip or tube.